U.S. patent number 4,085,614 [Application Number 05/639,776] was granted by the patent office on 1978-04-25 for vortex flow meter transducer.
This patent grant is currently assigned to The Foxboro Company. Invention is credited to John R. Curran, David A. Richardson, George E. Sgourakes.
United States Patent |
4,085,614 |
Curran , et al. |
April 25, 1978 |
Vortex flow meter transducer
Abstract
A vortex-shedding flow-sensing instrument comprising a
flat-faced vortex-generating plate integral with a downstream
sensor-bar having flat sides set back laterally with respect to the
rows of vortices shed from the edges of the vortex-generating
plate. The side surfaces of the plate are tapered inwardly at a
moderate angle, and extend downstream a short distance to rear
surfaces at right angles with respect to the direction of fluid
flow. The downstream end of the sensor bar comprises a tail piece
the side surfaces of which are tapered inwardly at a moderate
angle, and extend downstream a short distance to a rear surface
perpendicular to the direction of fluid flow. The sensor bar
carries a liquid-filled capsule having as side walls a pair of
flexible diaphragms effectively in the plane of the sensor-bar side
surfaces. These diaphragms are of moderately large area so as to
respond to a relatively large portion of the vortex energy. The
diaphragms transmit alternating vortex pressure pulses interiorly
of the capsule to a sensing element in the form of a ceramic disc
having piezo-electric properties. The resulting alternating voltage
signal developed by this element is coupled through lead wires to
an amplifier arranged to produce a flow signal suitable for
transmission over relatively long distances.
Inventors: |
Curran; John R. (Attleboro,
MA), Sgourakes; George E. (Millis, MA), Richardson; David
A. (Sheldonville, MA) |
Assignee: |
The Foxboro Company (Foxboro,
MA)
|
Family
ID: |
23839633 |
Appl.
No.: |
05/639,776 |
Filed: |
December 11, 1975 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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463301 |
Apr 23, 1974 |
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Current U.S.
Class: |
73/861.24;
73/861.47 |
Current CPC
Class: |
G01F
1/3218 (20130101); G01F 1/3263 (20130101) |
Current International
Class: |
G01F
1/32 (20060101); G01F 001/32 () |
Field of
Search: |
;73/194B,194VS |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2,229,583 |
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Nov 1973 |
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DT |
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4,610,233 |
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Mar 1971 |
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JA |
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Other References
Mair, "The Effect of a Rear Mounted Disk on the Drag of a
Blunt-Based Body of Revolution" in Aeronautical Quarterly, Nov.
1965, pp. 350-360..
|
Primary Examiner: Goldstein; Herbert
Attorney, Agent or Firm: Parmelee, Johnson, Bollinger &
Bramblett
Parent Case Text
This is a continuation, of application Ser. No. 463,301 Filed Apr.
23, 1974 now abandoned.
Claims
We claim:
1. Flow-metering apparatus of the vortex-shedding type
comprising:
a flow-pipe section adapted to be coupled into a flow system;
a vortex-generating body mounted in said flow pipe section;
means defining an enclosed chamber downstream of said body
including flexible diaphragm means as one wall of said chamber
arranged to respond to the pressure fluctuations of vortices shed
from said body and to transmit such pressure fluctuations into the
interior of said chamber;
sensor means located outside of said flow-pipe section, remote from
the process fluid flowing through the associated flow system;
said sensor means comprising an enclosed compartment;
a pressure-responsive piezo-electric sensor element in said
compartment;
the area of said sensor element being about 2.5 times the area of
said diaphragm means;
conduit means connecting said compartment with said enclosed
chamber; and
liquid filling in said conduit means, said chamber, and said
compartment, to transmit vortex pressure pulses from said flexible
diaphragm means to said piezo-electric sensor element to produce a
pulsating flow signal corresponding to the fluid flow rate.
2. Flow-metering apparatus of the vortex-shedding type
comprising:
a flow-pipe section adapted to be coupled into a flow system;
a vortex-generating body mounted in said flow pipe section;
means defining an enclosed chamber downstream of said body
including flexible diaphragm means as one wall of said chamber
arranged to respond to the pressure fluctuations of vortices shed
from said body and to transmit such pressure fluctuations into the
interior of said chamber;
sensor means located outside of said flow-pipe section, remote from
the process fluid flowing through the associated flow system;
said sensor means comprising an enclosed compartment;
a pressure-responsive piezo-electric sensor element in said
compartment;
said sensor element and said diaphragm means both being circular in
configuration;
the diameter of said sensor element being about 1.6 times the
diameter of said diaphragm means;
conduit means connecting said compartment with said enclosed
chamber; and
liquid filling in said conduit means, said chamber, and said
compartment, to transmit vortex pressure pulses from said flexible
diaphragm means to said piezo-electric sensor element to produce a
pulsating flow signal corresponding to the fluid flow rate.
3. Flow-metering apparatus comprising:
a vortex-shedding element to be positioned in a flowing fluid and
having laterally-spaced surfaces adapted to shed spaced rows of
vortices at a frequency corresponding to the fluid flow rate;
vortex-sensing means positioned downstream of said element between
said rows of vortices to produce a signal responsive to said
shedding frequency;
said vortex-sensing means comprising a structure having a
predetermined downstream dimension and defining an internal cavity
with side regions thereof facing said rows of vortices,
respectively;
first and second thin flexible diaphragms sealingly secured to said
structure adjacent and at least approximately parallel to said side
regions of said cavity, said diaphragms serving to fully enclose
and seal said cavity;
a liquid fill in said cavity supporting said diaphragms against
static pressure of said flowing fluid and providing for
transmission within said cavity of pressure fluctuations applied to
said diaphragms by said vortices;
a substantially rigid, pressure-responsive member mounted within
and across said cavity in a position at least approximately
parallel to said diaphragms to be subjected to said pressure
fluctuations, said member having an electrical characteristic which
varies with changes in the differential pressure appearing across
the surfaces of said member which face said diaphragms;
means mounting said member within said cavity and arranged to
provide for effective liquid sealing between the two regions of
said cavity on opposite sides of said member, whereby the movement
of said diaphragms in response to said pressure fluctuations is
effectively minimized so as to avoid fatiguing said diaphragms
during extended periods of usage of the apparatus; and
electrical transmission means connected to said member within said
cavity and leading out through said structure to provide for access
to the remote end of said transmission means from outside of the
flowing fluid for connection to means to develop a flow signal
responsive to the frequency of said changes of said electrical
characteristic.
4. Flow-metering apparatus comprising:
a vortex-shedding element to be positioned in a flowing fluid
stream to produce vortices at a frequency corresponding to the
fluid flow rate;
vortex-sensing apparatus for detecting the vortices in the fluid
comprising:
a structure providing sealed interior spaces;
first and second diaphragms forming part of said structure to
contact the flowing fluid and to transmit into said sealed interior
spaces pressure variations resulting from said vortices;
a liquid fill in said sealed spaces supporting said diaphragms
against static pressure of said flowing fluid and providing for
transmission within said sealed spaces of said pressure variations
applied through said diaphragms;
a substantially rigid pressure-responsive member mounted within
said interior spaces and having opposed sides subjected
respectively to said pressure variations developed by said first
and second diaphragms, said member having an electrical
characteristic which varies with changes in the differential
pressure appearing between said opposed sides due to said pressure
variations;
means mounting said member to provide for effective liquid sealing
between the two regions which are respectively adjacent said
opposed sides of said member, thereby to prevent shunting flow of
liquid around said member; and
electrical transmission means connected to said member to develop a
signal responsive to the frequency of said changes of said
electrical characteristic.
5. Flow-metering apparatus comprising:
a vortex-shedding element to be positioned in a flowing fluid
stream to produce vortices at a rate corresponding to the fluid
flow rate;
vortex-sensing apparatus for detecting the vortices in the fluid
comprising:
wall means defining sealed internal spaces;
first and second diaphragms forming part of said wall means to be
exposed to said flowing fluid and to transmit into said sealed
spaces pressure fluctuations caused by said vortices;
a liquid fill in said sealed spaces to support said diaphragms
against static pressure of said flowing fluid and to provide for
transmission within said sealed spaces of said pressure
fluctuations applied through said diaphragms;
a substantially-rigid piezo-electric member within said sealed
spaces and having opposed surfaces positioned to be subjected to
the differential pressure developed between said diaphragms by said
pressure fluctuations so as to produce an alternating voltage
signal reflecting the vortex-shedding frequency;
means fixedly mounting said piezo-electric member to provide for
effective liquid sealing between the regions adjacent said opposed
surfaces thereof to prevent movement of liquid past said
piezo-electric member, whereby said piezo-electric member provides
pressure-responsive sensitivity without requiring significant
movement of said liquid and said diaphragms; and
means to develop a flow signal responsive to the frequency of said
alternating voltage signal.
6. Apparatus as in claim 5, wherein said piezo-electric member
comprises an edge-supported disc.
7. Flow-metering apparatus of the vortex-shedding type
comprising:
a flow-pipe section adapted to be coupled into a flow system;
a vortex-generating body mounted in said flow-pipe section;
means near said vortex-generating body providing an enclosed
chamber and including means dividing said chamber into two separate
sections;
first and second diaphragm means forming part of said chamber
sections respectively and arranged to respond to the pressure
fluctuations of vortices shed from said body and to transmit such
pressure fluctuations into the interior of each chamber
section;
sensor means located outside of said flow-pipe section;
said sensor means comprising first and second enclosed
compartments;
conduit means connecting said first and second compartments with
said first and second chamber sections respectively;
liquid filling in said conduit means, said chamber sections and
said compartments, to transmit vortex pressure pulses from said
diaphragm means to the interior of each compartment;
said sensor means further comprising pressure-responsive means
sealingly mounted adjacent said compartments to receive at opposite
sides thereof the pressure pulses from said compartments
respectively;
said pressure-responsive means including means providing an
electrical characteristic which varies with changes in the
differential pressure appearing between said opposite sides due to
said pressure-pulses; and
electrical transmission means connected to said pressure-responsive
means to develop a signal corresponding to the frequency of
variation of said electrical characteristics.
8. Apparatus as in claim 7, wherein said pressure-responsive means
comprises a piezo-electric member.
9. Apparatus as in claim 8, wherein said piezo-electric member is
an edge-supported disc.
10. Apparatus as in claim 8, wherein the area of said
piezo-electric member is about 2.5 times the area of either of said
diaphragm means.
11. Apparatus as in claim 8, wherein said piezo-electric member is
circular in configuration, and both of said first and second
diaphragms means are circular in configuration with diameters of
equal size;
the diameter of said piezo-electric member being about 1.6 times
the diameter of said first and second diaphragm means.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to fluid flow measuring apparatus. More
particularly, this invention relates to flow meters of the vortex
shedding type, typically employed to measure the velocity of fluid
flow (either liquid or gas) through a pipe or other channel.
2. Description of the Prior Art
It has been known for many years that vortices are developed in a
fluid flowing past a non-streamlined obstruction. It has also been
known that with certain arrangements the vortices are developed by
alternately shedding at regular intervals from opposite edges of
the obstruction to form corresponding rows of vortices. Such
vortices establish a so-called von Karman "vortex street", which is
a stable vortex formation consisting of two nearly-parallel rows of
evenly-spaced vortices travelling with the flow stream.
In a von Karman vortex street, the vortices of one row are
staggered relative to those of the other row by approximately
one-half the distance between consecutive vortices in the same row.
The spacing between successive vortices in each row is very nearly
constant over a range of flow rates, so that the frequency of
vortex formation is correspondingly proportional to the velocity of
the fluid. Thus, by sensing the frequency of vortex shedding, it is
possible to measure the fluid flow rate.
Various proposals have been made for such flow measuring apparatus
of the vortex-shedding type, and some equipment has gone into
commercial use. Commonly, such apparatus comprises a rod-like
vortex-shedding obstruction positioned in the flowing fluid at
right angles to the direction of fluid flow. The obstruction has in
many suggested arrangements been a right-circular cylinder,
typically a relatively thin, elongate element as shown for example
in U.S. Pat. No. 3,564,915 (FIG. 4). Other shapes have been
proposed. For example, U.S. Pat. No. 3,116,639 (Bird), shows in
FIG. 10 an obstruction of triangular cross-section positioned with
one flat surface facing upstream. In like vein, U.S. Pat. No.
3,572,117 (Rodely) also shows the same triangular cross-section
arrangement, and additionally shows various other shapes comparable
to known configurations as disclosed for example in "Fluid Dynamic
Drag", published in 1965 by S. F. Hoerner (see particularly pages
3-7 and 3-17).
A number of different techniques have been proposed for detecting
the shedding vortices so as to devlop a flow signal responsive to
the shedding frequency. Thermal sensors of the so-called "hot-wire"
type (i.e., thermistors, hot films, etc.) frequently have been used
in vortex flow meters. The electrical resistance of such sensor
elements varies with changes in the cooling rate caused by the
passage of the vortices, or by changes in streamline velocity, and
this resistance variation is detected by measuring the
corresponding changes in current flow through the element.
Such thermal detectors have not been satisfactory for industrial
applications. The sensor elements typically are delicate and
subject to damage from wear or impact, and also are subject to
shorting-out from fluid leakage. A potential hazard is created
because the sensor elements must be heated to a temperature above
that of the flowing fluid, and because an electrical current must
be introduced into the sensor equipment. The output signal
generally is small and difficult to detect without highly complex
electronic circuitry.
In addition, the output signal appears as a change-in-level of a
non-zero current, and thus inherently presents a problem of
separating the variable component from the fixed signal level. The
output signal variation ordinarily is a small fraction of the fixed
signal level, and is particularly subject to noise due to cooling
effects from sources other than vortices, as well as being subject
to extraneous variations resulting from changes in ambient
conditions. Moreover, the output signal variations decrease with
increasing vortex frequency, and thus tend to be lost in noise
signals at the higher flow rates. Protective coatings on the sensor
element are generally quite thin in order to minimize this effect,
but this, in turn, results in undesirably low resistance to wear
from the flowing fluid.
Other types of detectors have been suggested in an effort to
overcome the deficiencies of thermal sensors. In one
vortex-detecting arrangement used commercially, a shuttle-like
element is mounted in a lateral passageway through the
vortex-shedding obstruction to be oscillated back-and-forth by the
pressure fluctuations of the passing vortices. The shuttle movement
is detected by a nearby pick-up coil to produce a signal reflecting
the frequency of vortex generation. The above-mentioned U.S. Pat.
No. 3,564,915 shows such an oscillating type of sensor using a ball
element (FIG. 7B). That patent also suggests (FIG. 7A) a
diaphragm-type device mounted in the center of a lateral bore
extending through a rod-shaped obstruction, but apparently does not
relate this to any particular sensor design.
As still another approach to the problem, the above-mentioned Bird
U.S. Pat. No. 3,116,639 shows a relatively thin vane-like sensing
element located downstream of the vortex-shedding obstruction,
positioned in alignment with the direction of fluid flow and
centrally located so that the spaced rows of vortices pass along
opposite sides thereof. This vane-like element is said to oscillate
rotatably in a twisting, torsional movement about an axis
perpendicular to the fluid flow direction, in response to the
pressure fluctuations of the vortices passing thereby. It is
proposed in the patent that the length of the vane, in the
direction of fluid flow, should be equal to the vortex spacing in a
row of vortices.
Various electrical transducer means are proposed in the
above-mentioned Bird U.S. Pat. No. 3,116,639 for detecting the
intended rotational movement of the vane, as by sensing with
conventional electro-magnetic means the oscillatory twisting motion
of a support shaft for the vane. This patent also puts forth the
notion that the vane it discloses might be made of a piezo-electric
material which is strained cyclically by the passage of the
vortices along its operative faces to produce an alternating
voltage. Piezo-electric means also are proposed to be used as a
fluid-fluctuation detectors in U.S. Pat. Nos. 2,809,520 and
3,218,852. None of these prior disclosures, however, shows a
practical flow meter arrangement, and developers of
commercially-offered apparatus have not attempted to adapt
piezo-electric devices to vortex flow meters, resorting instead to
other arrangements such as the thermal detectors described
previously herein.
The vortex flow-metering devices proposed heretofore have suffered
from important drawbacks. For example, certain types of such flow
meters have been generally complex and quite expensive to
manufacture, and thus have not been adaptable to many applications
where cost is a significant factor. Also, available vortex
flow-metering apparatus typically has not been adequately reliable
in operation, and particularly in some cases has not been capable
of satisfactory operation for measuring the flow rate of hostile
fluids, e.g. fluids containing dirt, corrosive liquids or gases, or
other potentially harmful material such as material which tends to
coat the surfaces of an object in the flow stream. Many vortex flow
meters have not been capable of sufficiently linear operation over
desirably wide ranges of fluid flow rates.
SUMMARY OF THE INVENTION
In an embodiment of this invention to be described hereinbelow in
detail, a flow meter of the vortex-shedding type is provided
comprising a unique plate-like vortex-shedding obstruction. This
plate is formed with a flat front face the side edges of which are
relatively sharp to aid in establishing clean-cut vortex
generation. The side edge regions of the plate are followed (in a
downstream sense) by respective inwardly-sloping side surfaces
terminating a short distance downstream, preferably, in certain
configurations, at flat rear surfaces parallel to the front face of
the plate, i.e. perpendicular to the direction of fluid flow. This
overall configuration has been found to provide desirably strong,
stable vortex shedding.
Further downstream, i.e. beyond the perpendicular rear surfaces of
the vortex-generating plate, is a bar-like sensor-carrying member
with flat, parallel side surfaces aligned with the direction of
fluid flow. The transverse spacing between the flat side surfaces
of this bar-like member is substantially less than the
vortex-generating plate. The straight side surfaces of the bar
appear somewhat as shelves which form, in cooperation with the
corresponding adjacent plate surfaces, cavity-like regions
providing for accommodation, without significant interference, of
the full extent of the vortices shed from the vortex-generating
plate edges.
This bar-like member, located downstream of the vortex-generating
plate, is formed in the region of the shelf-like side surfaces with
an interior chamber containing a vortex-sensing capsule sealed off
from the process fluid by flexible diaphragms at both side walls.
This sealed capsule contains a ceramic disc of the two-layered type
adapted to produce, by flexure-bending piezo-electric action,
electrical signals responsive to variations in pressure applied to
the side surfaces of the disc as a result of vortex shedding. The
interior of the capsule is filled with a fluid such as oil which
transmits pressures from the deflectable sealing diaphragms to the
side surfaces of the ceramic disc. As the vortices shed alternately
alongside the two opposed sealing diaphragms, the vortex pressure
fluctuations are transmitted through the oil-filling to the ceramic
disc which develops corresponding electrical output signals
indicative of the rate of fluid flow.
This transducer produces strong signals corresponding to flow rate,
and has a desirably high signal-to-noise ratio. The signal strength
actually increases with increasing vortex frequency, providing an
important advantage particularly in comparison to thermal sensors.
A sealed unit as described hereinbelow is resistant to damage, and
is capable of operation in hostile or dirty fluids. The sealing
diaphragms are so located as to minimize the chance of their being
damaged by suspended particles in the process fluid. The transducer
output signal is relatively clean, and can be processed without
expensive and complicated electronic circuitry such as
sophisticated filters or the like.
Accordingly, it is a principal object of the present invention to
provide superior flow measuring apparatus of the vortex-shedding
type. Other more detailed objects, aspects and advantages of the
invention will be pointed out in, or apparent from, the following
written description considered together with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a side elevation view, partly broken away, of a flow
meter constructed in accordance with principles of this
invention;
FIG. 2 is an elevation view of the integral flow-bar member which
is mounted in a flow-metering pipe section and readily removable
therefrom as a unit;
FIG. 3 is a horizontal section taken along line 3--3 of FIG. 2 to
show the outline configuration of the vortex-generating plate and
its associated sensor bar;
FIG. 4 is a front elevation view of the vortex generating plate,
showing the curved top and bottom edges adapted to match the curved
contour of the adjacent pipe section;
FIG. 5 is a horizontal section taken along line 5--5 of FIG. 2 to
show details of the oil-filled capsule providing a sealed sensor
element;
FIG. 6 is a vertical section through the central portion of the
sensor bar, taken along a plane containing the pipe axis and
showing the arrangement of the vortex-sensing capsule relative to
the sensor bar;
FIG. 7 is a detail section taken along line 7--7 of FIG. 6;
FIG. 8 is a detail perspective view showing the manner of making
electrical connection to the ceramic disc;
FIG. 9 is a perspective view showing a flapper-actuated
piezo-electric sensor;
FIG. 10 is a horizontal section taken along line 10--10 of FIG.
9;
FIG. 11 shows an arrangement wherein a piezo-electric element is
bonded to a flexible diaphragm of an oil-filled sensor unit;
and
FIGS. 12 and 13 show an arrangement wherein the detector element is
located outside of the flow-pipe.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to FIG. 1, a flow meter constructed in accordance
with principles of the present invention comprises a pipe section
10 adapted to be coupled by means of conventional end flanges into
a pipe system (not shown) carrying fluid the flow velocity of which
is to be determined. Mounted centrally in the pipe section 10, in
the path of the flowing fluid, is an elongate, upstanding body,
generally indicated at 12, serving as a composite vortex-generating
and sensing unit. This body 12 is disposed vertically, with its
longitudinal axis perpendicular to the direction of fluid flow (in
this case flowing from left-to-right). However, it should be
understood that a vertical disposition is not required, and that
for some applications a horizontal or other non-vertical position
of the body 12 will be preferred.
Referring now also to FIG. 2, the flow-metering body 12
advantageously forms part of an elongate, integral meter member 14
which passes vertically down through an opening in the top of the
pipe section 10. With such an arrangement, the body 12 is readily
removable from its operating position in the pipe section, as for
maintenance purposes, cleaning, or the like. The integral member 14
is held in place by a clamp arrangement including a cross-piece 16
fastened in conventional fashion to upstanding bosses 18 cast with
the pipe section. A crushed gasket 20 seals the pressure joint
between the member 14 and the pipe section wall.
Referring now also to FIGS. 3 and 4, the body 12 comprises a
vortex-generating plate 22 having a flat front surface 24 facing
upstream towards the oncoming fluid. The sides of the plate 22
desirably are formed with relatively sharp edges 26 which are
followed by flat side surfaces 28 extending downstream with an
inward taper at a moderate angle with respect to the direction of
fluid flow, e.g. a taper angle between about 5.degree. to
45.degree., and advantageously 30.degree. as shown. The
vortex-generating plate 22 preferably is relatively thin. Thus the
side surfaces 28 extend downstream only a relatively short distance
D, substantially less than the width W of the plate 22, and
preferably between about one-tenth and one-half of the width. In
the specific embodiment shown in the drawings, the side surfaces
terminate at flat rear surfaces 30 which are parallel to the front
face 24, i.e. perpendicular to the direction of fluid flow.
This configuration of the vortex-generating plate 22 has been found
capable of developing strong, stable vortices from the upstream
edges 26. These side edges ideally define sharp angles between the
front face 24 and the inwardly tapered side surfaces 28. However,
for ease of manufacture and good quality control, these edges can,
as shown, be flattened for a very short distance downstream without
significantly interfering with the development of the desired
vigorous vortices.
In accordance with one important aspect of the present invention,
the strong vortices shed by the edges 26 of the plate 22 are sensed
directly inside the wake of the turbulence immediately downstream
of the plate, by means positioned between the two rows of vortices.
For this purpose, directly behind (i.e. downstream of) the
vortex-generating plate 22 there is positioned a generally
rectangular bar-like sensor-carrying member 40 having parallel side
surfaces 42 aligned with the direction of fluid flow, i.e. the side
surfaces are perpendicular to the rear surfaces 30 of plate 22.
Referring also to FIG. 5, this bar presents to the vortices shed
from the side edges 26 a unique pressure-transducer capsule
arrangement, generally indicated at 44, which is peculiarly well
adapted for sensing the vortex pressure fluctuations, as will be
described hereinbelow in detail. The thickness T of the bar 40
(FIG. 3) is in the embodiment disclosed substantially less than the
lateral dimension "w" between the outer edges of the rear surfaces
28, e.g. about one-half of that dimension. The arrangement of
surfaces 28, 30 and 42 advantageously establishes recess-like
pockets or cavity regions 46, on opposite sides of the bar, which
accommodate the desired vigorous development and free, unimpeded
passage of the shed vortices without significant degradation
thereof as they pass down alongside of the sensor bar 40. These
cavity regions moveover provide a suitably noise-free area for the
pressure detector 44.
In the preferred embodiment, the sensor bar 40 is fabricated as an
integral part of the flow meter member 14, being constructed
therewith as part of the manufacturing operations. The
vortex-generating plate 22, although formed separately, is secured
to member 14, as by means of conventional machine screws or welding
(not shown), so as to be an effectively integral part of member 14.
However, it should be noted that this contiguous physical
relationship is not essential to the performance of the flow meter,
from an operational point of view. More specifically, the bar 40
may be spaced away from the rear surfaces of the plate 22, at least
a short distance downstream, and the two components may be
separately supported within the pipe section 10.
The downstream end of the sensor bar 40 (FIGS. 3 and 5) is formed
with a tail piece 48 having side surfaces tapered at a moderate
angle down to a flat, perpendicular rear surface. It has been found
that the tapered configuration of this part of the bar is effective
in providing good linearity between the changes in vortex-shedding
frequency and the corresponding changes in fluid flow rate.
Preferably, the angle of taper is about 30.degree. with respect to
the direction of fluid flow. The tail length TL and the tail width
TW should be considerably smaller than the overall bar length L,
desirably less than one-half and preferably less than one-quarter
of that length.
Referring now also to FIGS. 6 and 7, the special sealed transducer
44 is located in an interior chamber 50 of circular cross-section
and extending completely through the bar 40. The transducer capsule
comprises a pair of thin (0.003 inch), flexible, circular metal
diaphragms 52, on opposite sides of the bar, adapted to seal an
internal transducer element 56 (to be described below in detail)
from the process fluid. These diaphragms transmit the vortex
pressure energy into the interior of the chamber 50 to actuate the
transducer element.
This flexible, area-type diaphragm arrangement provides a
relatively large sensing area to be exposed to the vortex pressure
fluctuations, ensuring that a suitably large proportion of the
total vortex energy is made available to the transducer element,
and tending to minimize the effects of noise by cancellation due to
averaging over the entire area. Thus the diaphragm area should,
from that point of view, be maximized. A circular diaphragm
configuration is presently preferred for ease of manufacture, and
also for ease of sealing the peripheral edges of the internal
transducer element so as to maximize the pressure loading on that
element. This sealing allows the transducer element to accept a
greater proportion of the pressure signal, thus reducing fatigue
effects in the diaphragms as well as increasing the available
signal level.
There is no necessary limitation of the broad invention to circular
diaphragms or transducer elements, and for some applications
rectangular or other area configurations can be used with
advantage, particularly for the purpose of maximizing the total
area subjected to the vortex pressure fluctuations. The downstream
dimension of the diaphragm (e.g. the diameter of a circular
diaphragm) should be smaller than the spacing between successive
vortices of either row, and preferably should be less than half of
such spacing in order to avoid reduction of signal due to pressure
pulses being applied simultaneously to both opposed diaphragms.
However, within these constraints, the downstream dimension should
be as large as possible and preferably at least one-tenth of the
vortex spacing.
In the described embodiment designed for use in a pipe having a 3
inch internal diameter, the sensor bar 40 had an overall length L
of 0.923 inch, with a chamber 50 having the maximum possible
diameter (about 0.63 inch) which can be formed within the
flat-sided regions of the bar 40 and accommodate diaphragms 52
having the maximum possible diameter of about 0.75 inch. In such a
meter, the spacing between successive vortices in one row of the
vortex street would be within the range of around 2.5 inches to 3.5
inches, so that the downstream dimension of the diaphragm was
approximately one-quarter of the vortex spacing, or "wavelength" of
the shed vortices.
The pressure fluctuations of the vortices shed from plate 22 are
transmitted through the diaphragms 52 to a transducer element 56
which in this embodiment is a circular disc or wafer 56 of thin
(0.021 inch) ceramic material having piezo-electric properties. The
disc comprises two layers separated by a thin vane of electrically
conductive material, e.g. brass (not shown); however, other types
of piezo-electric elements can be used. The outer disc surfaces are
covered with a thin film of silver (not shown) for making good
electrical connection to the ceramic material so as to pick up the
electrical signals developed by the disc in response to the applied
pressure fluctuations.
Ceramic discs as described herein are available commercially, under
the name Bimorph, from the Vernitron Corporation of Bedford, Ohio.
The ceramic is edge-supported, preferably with a simple support
comparable to a knife-edge or the equivalent, although a more
conventional cantilever-type support is functional even though not
as efficient. The applied pressure flexes the ceramic material
(i.e. it operates in the so-called "flexure mode") which responds
by generating corresponding positive and negative electrical
charges at its opposite surfaces. A detailed discussion of the
operational principles of such a device are set forth in an article
by C. P. Germano in Volume AU-19, Number 1, of the March, 1971
issue of "IEEE Transactions on Audio and Electroacoustics", pages
6-12.
Electrical contact with the silver coating on the sides of the
ceramic disc 56 is effected by extremely thin copper rings 60 (see
also FIG. 8) at both sides of the disc. These copper rings are
bonded to and carried by thin layers of sheet plastic insulating
material 62. At one circumferential point of each ring 60, the
conductive material is extended out radially to corresponding
copper leads 64 which pass up through a small vertical bore 66 (see
FIG. 2) in the flow metering member 14. These leads are insulated
by plastic sheathing bonded thereto, and are connected at their
upper ends to respective terminals of an insulating glass-to-metal
seal 68 of known construction. The leads 64 continue from the seal
terminals to a weather-proofed enclosure 70 where they connect to a
hermetically sealed amplifier 71A having a pair of output leads 65.
These output leads deliver a d-c output signal to a terminal box
section 71B of the enclosure 70. The amplifier 71A comprises
circuitry for developing an output signal adapted to be transmitted
over relatively long distances. Such circuitry does not form a part
of the present invention, and thus will not be described herein in
detail.
Surrounding the outer periphery of the ceramic disc 56 is a plastic
insulating ring 72 made, for example, of a high-temperature plastic
such as that known commercially as "Astrel". This plastic ring is
forced outwardly into a tight liquid-sealing fit against the inner
wall of the chamber 50 by the radial pressure of a pair of metal
spacer rings 74 on opposite sides of the ceramic disc 56. The outer
diameter of these spacer rings is slightly larger than the inner
diameter of the insulating ring 72, and the spacer rings are
pressed in place by sufficient force to expand the insulating ring
slightly so as to establish the desired liquid-tight seal. The
spacer rings are held in place by respective circular clamp plates
76 which are secured in conventional fashion such as by staking at
the outer circumference points, or by a retainer ring arrangement
(not shown). As shown in FIG. 6, these clamp plates are notched in
spaced locations around their outer periphery, to permit the flow
of liquid thereby.
The interior spaces of the chamber 50 are filled with oil 78
injected, for example, through oil ports 80 and 82 respectively
above and below the chamber. The upper port 80 communicates with
the vertical bore 66 (FIG. 2), and the oil inserted through that
port fills the bore 66 and also flows down to the insulating ring
72. At the point where bore 66 enters the interior chamber 50, the
insulating ring is formed, at its outer circumference, with a
transverse passage 84 (see FIG. 7) providing for flow of the oil
into the right-hand side of the chamber, passing through notches of
the corresponding clamp plate 76 to the region between the
right-hand diaphragm 52 and the ceramic disc 56. The oil from the
lower port 82 flows up through a bore 88 to the insulating ring 72
where a transverse passage 90 in that ring carries the oil to the
left, through the corresponding clamp plate 76, and into the spaces
between the left-hand diaphragm 52 and the ceramic disc 56.
The oil fill 78 on both sides of the ceramic disc 52 serves to
transmit to the disc the pressure fluctuations applied to the outer
surfaces of the diaphragms 52 by the passage of the vortices shed
by the plate 22. The ceramic disc is thereby flexed at the
frequency of the passing vortices, and develops, in response,
corresponding electrical pulses indicating, by the frequency of
occurrence thereof, the flow rate of the fluid being monitored. The
oil fill provides a desirably benevolent environment for the
ceramic material, as well as for the other components of the
transducer capsule. It also aids in filtering out noise pressure
components appearing in the flowing fluid, reducing the need for
electronic filtering of the transducer output signal.
Referring again to FIG. 8, the copper ring 60 on the near side of
the disc 56 is interrupted at 94 to provide a small (0.005 inch
wide) gap through which the oil fill 78 can flow, very slowly,
whenever necessary to equalize the pressures on opposite side of
the disc. Such pressure differential can build up as a result of
ambient temperature variations, for example. The gap 94 is so small
that essentially no oil can flow through it in response to the
relatively rapid changes in pressure resulting from the vortices
passing alongside the diaphragms.
With the above-described arrangement, the ceramic disc 56 is
conductively insulated (electrically) from the metal body 12, from
the flowing fluid, and from the pipe section 10. The signal
produced by the transducer capsule is generated internally by
piezo-electric action, so that no electrical energy need be
directed into the interior of the body 12 or the flow system. The
piezo-electric transducer not only develops a relatively large
flow-responsive signal, but also is advantageous in that it does
not introduce a permanent (long time) d-c component into the flow
signal, even if the ceramic is physically offset in some manner.
The transducer moreover is responsive only to differential pressure
fluctuations, and not to static pressure variations or cooling
effects from any source as when using thermal sensors. Thus the
flow signal of the present transducer is relatively clean and more
readily processable.
Vortex meters as described herein can be sized to suit different
flow pipe diameters. The following Table I illustrates dimensional
ranges which presently appear appropriate for pipe sizes of 2
inches, 3 inches and 4 inches, respectively. The dimensional
reference symbols (W,L, etc.) shown in Table I correspond to those
used with FIG. 3 of the drawings herewith.
Table I ______________________________________ VORTEX SHEDDING BODY
DIMENSIONS (ALL DIMENSIONS IN INCHES) Pipe Size W D L T TW TL
______________________________________ 2" 0.564 0.240 .776 0.254
0.097 0.136 3" 0.952 0.238 .923 0.381 0.143 0.206 4" 1.250 0.312
.988 0.500 0.188 0.270 ______________________________________
Referring now to FIG. 9, a flow metering arrangement is shown
comprising a vortex-generating plate 22, as previously described,
and a sensor bar 40 having an outline configuration as previously
described, but with a different internal sensor arrangement. More
particularly, the bar 40 is formed with a generally rectangular,
elongate internal chamber 100, parallel to the longitudinal axis of
the bar (i.e. extending diametrically with respect to the pipe
section).
Within the rectangular chamber 100 is an elongate strip 102 of
piezo-electric material such as previously described, extending
substantially the full length of the chamber 100 and centrally
positioned with respect thereto. At its upper end, this
piezo-electric element is held rigidly by the pressure grip of a
conventional clamp plate arrangement 104, to provide for
cantilever-supported bending of the element 102 about an axis
parallel to the direction of fluid flow.
At the side walls of the sensor bar 40 are respective elongate,
rectangular flapper plates 106 of thin, somewhat flexible metal.
These plates are formed at the upstream edges thereof with integral
tabs 108 which bend inwardly to a region of pivotal engagement with
the sensor bar 40. The plates 106 are fastened together
mechanically by a pin 110 at the lower, downstream region of the
plates. These plates serve in effect as flexures, bendable about
the flexure points established by the tabs 108, in response to
transverse forces due to pressure fluctuations from the vortices
shed by vortex-generating plate 22.
The pin 110 which secures the flexure plate 106 together also is
fastened to the lower end of the piezo-electric strip 102. Thus, as
the flexure plates 106 move back and forth laterally (transversely
between the two rows of shed vortices) due to alternating pressure
fluctuations of the shed vortices, this movement is transmitted to
the piezo-electric element 102 which develops a corresponding
electrical output signal. This electrical output signal is
conducted from the sensor element 102 by an electrical lead
arrangement (not shown) as previously described, in order to
present the piezo-electric signal to suitable processing means such
as an electronic amplifier, etc.
It may be noted that the flapper plates 106 have a very substantial
area subject to the vortex pressure fluctuations. That is, the
downstream dimension of these plates is effectively equal to the
side wall dimension (parallel to the fluid flow direction) of the
sensor bar 40, and the longitudinal dimension is substantially
equal to the axial dimension of the vortex-generating plate 22.
Thus, a substantial portion of the available vortex energy is
coupled to the the piezo-electric element 102.
The flexure plates 106 are spaced a short distance away from the
side walls of the sensor bar 40, and the flowing fluid fills the
interior of the chamber 100. Thus, this arrangement is suitable for
cryogenic applications, where the fluid temperature would be so low
as to preclude the use of an oil-fill as previously described with
reference to the FIG. 7 arrangement.
FIG. 11 shows another arrangement having a sealed diaphragm capsule
with an oil-fill. In this arrangement, a piezo-electric element
120, such as previously described, is insulatedly bonded directly
to the internal surface of one of the sealing diaphragms 122. Thus
the sensor element provides the primary spring-force reaction to
the vortex pressure pulses, so that more of the vortex energy is
directed to the sensor element, thereby producing a signal
improvement. Also, the fatique effects in the metal diaphragm 122
are correspondingly reduced.
The inner section 124 of the capsule is formed with small holes 126
to permit flow of the oil-fill 128 from one side to the other, to
provide for optimal damping of the dynamic response
characteristics. Suitable means for making electrical connection to
the sensor element can be provided as previously described. For
some applications, it may be desirable to bond piezo-electric
sensors to both of the diaphragms.
Referring now to FIGS. 12 and 13, there is shown a vortex-shedding
flow-meter arrangement comprising a cast integral member 150
consisting of a flow section 152, adapted to be coupled into a
flow-pipe system, and an outer weather-proofed enclosure 154
containing certain operating elements to be described. Within the
flow channel defined by the flow section 152 there is mounted an
elongate vortex-shedding body 156, welded or otherwise secured in
place, which is formed, e.g. during extrusion, with a
cross-sectional configuration as previously described with
reference to FIG. 3 herein.
This elongate body 156 includes a front plate 158 and an integral
trailing sensor-bar 160 having identical circular sealing
diaphragms 162, 164 on opposite sides thereof. These diaphragms
enclose an oil-filled interior chamber 166 of circular
cross-section and having a divider partition 168 establishing two
separate chamber sections 170, 172, each adjacent a corresponding
diaphragm. As the shed vortices flow past the bar 160, the
resulting pressure fluctuations are transmitted through the
diaphragms 162, 164 into the corresponding oil-filled chamber
sections.
These pressure fluctuations in the chamber sections 170, 172 are
transmitted through respective oil-filled bores 174, 176 extending
longitudinally through the sensor bar 160 to the outer enclosure
154. In that enclosure, these bores 174, 176 connect respectively
to two oil-filled circular compartments 178, 180 located on
opposite sides of a circular edge-supported piezo-electric detector
element 182 such as the ceramic disc previously described
hereinabove. The vortex pressure pulses transmitted into these
compartments create alternating bending stresses in this
piezo-electric element, causing it to generate corresponding
voltage signals between its outer side surfaces. These voltage
signals are directed through suitable leads 184 (such as previously
described but for simplicity not shown in detail in FIGS. 12 and
13) to signal processing apparatus in the weather-proofed enclosure
154, including electronic amplifiers and the like, arranged to
develop a relatively high-powered flow signal for transmission to a
remote control room, etc.
The arrangement of FIGS. 12 and 13 has several significant
advantages. First, the piezo-electric element 178 is located
remotely from the process fluid flowing past the pressure-sensing
diaphragms 162, 164, and is thereby effectively isolated from that
fluid, particularly with respect to temperature effects. For
example, a typical piezo-electric element limited to operation at
temperatures no greater than 300.degree. F can, in the FIG. 12
arrangement, perform satisfactorily even though the process fluid
temperatures are much higher than 300.degree. F, e.g. perhaps as
high as 600.degree. F or so, i.e. at or near the maximum
temperature to which the oil-fill liquid can be subjected without
adverse effects.
Also, by physically separating the diaphragm chamber sections 170,
172 from the detector compartments 178, 180, it is possible to
selectively control the ratio between the area of the diaphragms
162, 164 and the area of the piezo-electric element 178. In that
regard, it has been found that a desirably high efficiency in
piezo-electric signal generation can be achieved by dimensioning
the various parts so that the diameter of the piezo-electric
element 178 is 1.6 times the diameter of the diaphragms 162, 164,
thus providing an area ratio of about 2.5:1.
The FIG. 12 arrangement also is advantageous in that the entire
vortex-shedding flow-meter apparatus can be supplied as an integral
structure, suitable for installation into a flow system without
difficulty, and operable as a composite unit to produce an
appropriate flow signal for transmission to a remote region.
Advantageously, the axial dimension of such unit is relatively
short. For example, the length of section 152 may be only 10% or
20% longer than the downstream length of the body 156, providing of
course that such dimensions are compatible with other perfromance
characteristics required of the overall system.
Although preferred embodiments of the invention have been described
hereinabove in detail, it is desired to emphasize that this is for
the purpose of illustrating the invention and thereby to enable
those skilled in this art to adapt the invention to various
different applications requiring modifications to the apparatus
described hereinabove; thus, the specific details of the
disclosures herein are not intended to be necessary limitations on
the scope of the present invention other than as required by the
prior art pertinent to this invention.
* * * * *